Providing thermal comfort in schools has a significant impact on students' performance and health. Climate change may result in an increased risk of overheating. A clear understanding and definition of overheating in schools is needed. Overheating can occur for various reasons. In order to deal with the problem realistically, it is necessary to have the correct design benchmarks. The UK government has recently published new overheating guidelines for schools. A comparison is made between the existing and new overheating guidelines. This is based on an analysis of recorded temperature data from 140 classrooms in 18 naturally ventilated primary schools in London, UK, which is then compared with records of occupants' thermal comfort responses to indoor temperature. It was found that the old guidelines were too lenient, thereby allowing some overheating to occur. The new guideline is more stringent, but needs further development to reflect occupants' perceptions more accurately.